The excitation and collection characteristics of optical probes used in clinical tissue laser-induced fluorescence measurements were investigated using Monte-Carlo simulations. The three basic optical characteristics were investigated: (i) effect of probe numerical aperture (ii) effect of excitation-collection separation (iii) effect of a quartz optical shield added to the tip of the probe. By varying the above three characteristics, the radial dependence of the fluorescence collected on the tissue surface was measured, along with the depth in tissue from which the light originated. The results indicate that the fluorescence penetration depth increases with excitation-collection separation.